This invention relates to a reusable, externally applied thermal protection system for use on aerospace vehicles subject to high thermal and mechanical stresses, and more particularly to a shell tile structure which effectively separates its primary functions as an insulator and load absorber.
Because space vehicles are subject to temperature extremes during ascent and re-entry, it is customary to provide the vehicle with a heat shield designed to protect the vehicle metallic substructure. The advent of the Space Shuttle initiated the need for a reusable, nonablative thermal protection system (TPS). The orbiter vehicle basically utilizes a conventional, skin-stringer aluminum aircraft structure. The properties of aluminum, however, dictate that the maximum operating temperature of the substructure not exceed 350.degree. F. In addition, the vehicle is subjected to multiple aerodynamic loads during flight, including aerodynamic pressure gradients and shocks, buffet and gust loads, acoustic pressure loads caused by boundary layer noise and concomittant substructure motion. Therefore, the TPS used must protect the aluminum substructure from high surface temperatures and at the same time withstand the thermal cycles and environmental loads of space flight.
The ceramic tile utilized on the initial flights of the shuttle, however, has relatively low strength and a low coefficient of thermal expansion as compared to metals. The relatively low tile strength precludes use for load bearing applications and dictates that the tiles be secured to the protected structure by a means which will minimize transfer of strains from the metal structure to the tile. Because of its homogeneous structure and brittle character, the ceramic tile has a strain to fracture performance considerably below the yield strain of aluminum and, as a result, must be monitored carefully for surface erosion, fraying and cracking. In addition, its low coefficient of thermal expansion is a deterrent to the tile ability to protect against gap heating.
It is preferable, therefore, to design a TPS which effectively separates its functions as an insulator and load absorber. This separation of functions allows flexibility in designing a TPS which can withstand thermal and mechanical stresses more effectively, without adding to the weight of the vehicle.
Accordingly, it is an object of this invention to provide an improved thermal protection system for aerospace vehicles.
Another object of this invention is to provide a reusable, externally applied thermal protection system which effectively separates its functions as an insulator and load absorber.
Another object of this invention is to provide a durable thermal protection system which utilizes a structurally strong outer shell to assume loads and a lightweight, thermally efficient insulation internally to resist heat flow.
Another object of this invention is to provide a thermal protection system which substantially reduces gap heating of the aerospace vehicle structure.
Still another object of this invention is to provide a thermal protection system which can be readily altered to meet desired thermal and mechanical performance.
Yet another object of this invention is to provide a thermal protection system which is strong in shear and tension, thereby permitting it to be directly connected to the substructure.
Still another object of this invention is to provide a thermal protection system which has a low parts count and is easily manufactured, installed, replaced and repaired.